1. Field of the Invention
The present invention relates to a method, system and program for halftoning input data to produce output values capable of being rendered by a multitone output device, such as a multitone printer.
2. Description of the Related Art
Digital images comprise pels or points of data that have one of many values representing a color or shade of gray value for that pel. For instance, many digital images may be expressed using 256 different intensities of gray, including black and white. However, laser printers are typically only capable of printing in black or white, or two different color values. To create the appearance of different shades of gray, prior art bi-level (i.e., black and white) laser printers utilize dithering algorithms. A dithering algorithm creates the illusion of new colors and shades by varying the pattern of dots. In this way, different shades or intensities of gray are produced by varying the patterns of black and white dots. In such bi-level printers there are no gray dots. However, there are output devices that include the full range of intermediate values and do not need halftoning, such as computer display monitors that display 16 million colors.
One common form of a dithering algorithm involves the use of a threshold matrix of values with which the values of picture elements or pels in the image are compared. The same matrix pattern is employed repeatedly to cover the image. The binary output value, i.e., black or white, for the bi-level printer is based on a comparison of whether the pel gray scale value is greater or less than the threshold value. The commonly assigned patent entitled “Clustered Aperiodic Mask”, having U.S. Pat. No. 5,917,951 provides techniques for creating the threshold matrix.
The above techniques are intended for dithering gray scale data for a bi-level printer. However, multitone printers can print at least some intermediate levels between white, i.e., nothing printed, and the full saturation value, i.e., black. There is thus a need in the art for providing halftoning algorithms for use with multitone printers to provide improved results when images are halftoned and printed on multitone printers.